A large majority of motor vehicle exhaust manifolds are made from conventional cast iron for strength and durability and other reasons. However, these manifolds are heavy and therefore detract from the fuel economy of the vehicle, as well as not being able to be easily fabricated into different shapes as with a sheet metal manifold. A further disadvantage is the motor vehicle underhood packaging constraints of cast iron manifolds.
A solution to the above is to provide a thin tubular, fabricated exhaust manifold out of a sheet metal that is light in weight and complies more easily with underhood packaging constraints. A disadvantage of sheet metal manifolds, however, is their tendency to transmit objectionable noise levels into the vehicle passenger compartment because of their thin wall nature.
The present invention overcomes the above disadvantage, or objection, by reducing the noise emanating from a sheet metal manifold without sacrificing the desired features of such a manifold. More particularly, the invention provides an outer coating onto the shell of the manifold that is irregular in surface and provides a high, or large surface area that damps the ability of the thin sheet metal manifold shell to transmit noise, or sound waves.